This invention relates to a method and apparatus for sputter-etching a substrate. This invention is specifically suitable to etching a substrate of silicon or its compound during manufacture of an integrated circuit.
For purely chemically etching a substrate of silicon or its compound, use has been made of a mixture of fluoric and nitric acids, fluoric, nitric, and acetic acids, or the like. The etchant presents a problem of pollution. In addition, chemical etching complicates the process of manufacture of integrated circuits because the silicon wafers must repeatedly be placed into a vacuum for vacuum evaporation or sputtering and then removed therefrom for the chemical etching.
Use has also been made of conventional radio-frequency sputtering apparatus, which removes the problems of pollution and complicated processes. The speed of etching, however, is low. In addition, the sputtering apparatus is not suited to etching substrates with high reproducibility and with uniform depth of etch at various portions of a substrate because the r.f. sputtering apparatus has been developed specifically for deposition of a thin film on a substrate by sputtering and not for sputter-etching.
Recently, apparatus called a plasma asher has been put on the market. With the plasma asher, a radio-frequency coil produces plasmas by radio-frequency induction, which plasmas etch the substrate. Applicants have tried such apparatus to sputter-etch silicon wafers by producing plasmas in tetrafluoromethane introduced into the exhaustible space of the apparatus. It has been found that the wafer surface becomes rough and that the speed of etching is not uniform across the surface. For example, the etching was about twice as deep at the center of a silicon wafer, 5 cm in diameter, as at the periphery. In addition, the speed of etching irregularly depends on the distance between the wafers which should be placed in the space so as to be perpendicular to the coil axis.